I have a strange behaviour with Android 9, on launching the app for first time after installing from Play Store. On launching the app second time, the issue just disappear.
So, there are 2 issues, which can alternate, but they are always there:
1- the cookie manager is not able to read all cookies from a WebView. I have already posted a question on this subject here: xamarin android 9 cookies read issue
2- Firebase returns a blank notification token, even if the authorization token is valid and the google-service.json are good. In fact, in android 7 and 8 the firebase notification token is alway right.
To my test I use a Google Pixel 2 XL and I use the Xamarin framework.